Document FR 2 545 538 illustrates this prior art situation.
The OFD and the IFD present a segment surrounding the high pressure compressor, which is itself enclosed in its outer casing.
Thermal and mechanical optimization of the structures of the engine often lead to seeking a high level of stiffness for the high pressure compressor in order to guarantee small clearances at the tips of the blades.
It is therefore necessary to find a compromise between requirements for stiffness and requirements for low weight. Thus, increasing the thickness of the outer casing leads to an increase in its weight, so this technique for increasing the stiffness of the casing rapidly reaches its limit.
This compromise is particularly difficult to find with small engines (fan diameter less than 40 inches or 1 meter) with a compressor that is of the axial-centrifugal type. Under such circumstances, a portion of the outer casing of the high pressure compressor has a radius that is relatively large, such that the thickness of the outer casing has a major impact on the second moment of area or moment of inertia of the engine.